This invention relates generally to systems and methods for monitoring a machine tool to detect tool breakage and/or wear which produce changes in machining conditions that occur over a period of time.
Various machine tool monitoring systems and methods for sensing tool breakage or wear in lathes, milling machines and other machine tools based upon recognition of characteristic signatures in cutting noise vibrational signals have been developed and are well known. Generally, these systems and methods sense a broad band of energy produced during a machining or cutting process, detect the energy produced in a particular narrower high frequency band, and produce a corresponding analog electrical signal. This analog signal may be processed to provide a unipolar signal which has a level that is closely related to the cutting energy and to the efficiency of the cutting process. The unipolar signal may be analyzed in real-time for characteristics or clues that indicate tool breakage, excessive tool wear, or other events of interest in controlling the machining process. One clue which is often detected and which is sometimes used in conjunction with other clues from vibration or other sensed signals is a gradual decrease in the energy in the detected high frequency band of the unipolar vibration signal which accompanies a decrease in the efficiency of the cutting process. Events which have been found to produce relatively gradual decrease in the efficiency with which a tool cuts metal include the very rapid wear or "crumbly breakage" of ceramic lathe tools which takes place over a period of one to a few seconds of cutting time, the normal gradual wear of a turning and milling tool which takes place over a period of minutes of cutting time, or chip burning and tool clogging of milling tools which takes place over a period of time of the order of ten seconds or so. Events such as these all require changes in the machining operation, and all tend to be characterized by an increase in the total energy supplied by the machine tool and a decrease in the effective cutting energy, i.e., a decrease in cutting efficiency.
Data indicates that vibration energy at frequencies above about 30 KHz decreases as cutting efficiency decreases, whereas vibration energy at frequencies in the audio range tends to increase. The exact crossover frequency has not been determined. This decrease in high frequency energy may be detected and used for indicating the need for changes in a machining operation by creating and monitoring the level of a tracking mean value signal which tracks the mean value of the signal samples of the unipolar signal. A decrease in cutting energy may be detected by monitoring the tracking mean value signal and detecting the level of the signal decreasing to one or more predetermined threshold levels.
Mean signal level decreases, however, may result from other than tool breakage or wear. They may be caused, for example, by certain conditions that may be encountered as the tool path is followed during normal machining conditions, as where the tool load changes without a change in cutting efficiency. Furthermore, as the cutting tool wears and the tool cutting edge deforms, new sources of vibration may come into play due to abnormal cutting additions. These may be manifested as spiky noise which is superimposed upon the vibration signal. As the cutting changes, for example, from a chisel-type operation to more of a scraping-type action as the cutting tool dulls and the geometry of its cutting edge changes, there is often a marked increase in dense high amplitude noise spikes in the high frequency vibration signal. These noise spikes cannot be separated from the main cutting noise components of the vibrational signal. They tend to increase the mean signal level and obscure the decrease in the mean which would otherwise occur if the spikes were not present. As abnormal cutting conditions increase and cutting becomes more inefficient, the energy in the noise spikes, which represents mostly wasted energy, increases while the energy between the spikes, which represents effective cutting energy, decreases. Including the spike energy in the mean signal level measurement accordingly reduces the sensitivity of the cutting tool break/wear detector.
It is desirable to improve the sensitivity of cutting efficiency monitors and to provide a method and system which overcome the foregoing and other disadvantages of known methods and systems for detecting tool break and wear. It is to this end that the present invention is directed.